Continuous size reduction in motors and miniaturisation in electronic devices and systems in combination with increasing power result in an increase of the generated heat flux. There is therefore a need for better heat dissipation for efficiency, performance and reliability of these devices.
The growing importance of thermal management in machines and systems such as electrical motors, generators and transformers, electric vehicle batteries, electronics, telecoms, capacitors, lighting, wind turbines has brought the need to improve heat transfer for better heat removal.
There is a need for polyester film with electrical insulation and also improved thermal conductivity properties in order to provide a heat transfer path between components and therefore enable better heat dissipation. Non limiting applications include: slot, layer and phase insulation in electrical motors, generators, transformers; capacitor dielectrics, liner or substrates in electronic components and devices such as printed circuit boards, LEDs, thermal interface materials, thermally conductive tapes, pads, heat sinks, heat spreaders etc.
There is thus a need to provide biaxially stretched polyester films which combine electrical insulation and improved thermal conductivity properties with respect to known biaxially stretched polyester films, in particular with respect to biaxially oriented polyethylene terephthalate films, such as Mylar®, Melinex®, polyethylene naphthalate films, such as Teonex® and the like. More specifically, an increase in thermal conductivity in the direction perpendicular to the film plane is sought.
The addition of anisotropic particles with high thermal conductivity properties can effectively increase the thermal conductivity of such polyester films. However, biaxially stretching a film comprising a polymer matrix and anisotropic thermally conductive filler particles typically results in the orientation of said particles in a direction parallel to the film plane, as described in U.S. Pat. No. 7,494,704 B2, thus conferring an increase in thermal conductivity to the polyester film in the direction of the film plane. There is therefore also a need to provide a process for preparing a polyester film in which the anisotropic thermally conductive filler particles are not oriented in a direction parallel to the film plane, their addition resulting in increasing the thermal conductivity of said polyester film in the direction perpendicular to the film plane.
Document JP 2011-165792 discloses a multilayer biaxially stretched film comprising:                a heat conductive layer comprising a fibrous carbon material        one or two electrically insulating layers placed on one face or on each face of the heat conductive layer.        
The weight of the fibrous carbon material preferably ranges from 20 to 30% of the weight of the heat conductive layer. At such a high content of carbon material, the heat conductive layer may not by itself exhibit electrical insulation properties. Therefore, the electrically insulating layer(s) is (are) placed on one face or on each face of the heat conductive layer in order to impart electrical insulation properties to the biaxially stretched film. The electrically insulating layer(s) may further contain filler particles in an amount of less than 1% by weight.
Document JP 2013-038179 discloses a biaxially stretched film comprising from 2 to 20 weight % of a fibrous carbon material. As in document JP 2011-165792, one or two electrically insulating layer(s) is (are) placed on one face or on each face of the biaxially stretched film in order to impart electrical insulation properties to the film. The electrically insulating layer(s) may further contain filler particles in an amount of less than 1% by weight.
These two documents disclose co-extruded films in which an electrically conductive layer is rendered insulating by the addition of one or two electrically insulating layer(s).
Document JP 2011-129759 discloses a film for reinforcing a flexible printed board circuit. This film is said to exhibit excellent heat dissipation properties. It is made of a polyethylene naphthalene (PEN) matrix in which filler particles are embedded. The filler particles are present in the matrix at a content ranging from 10 to 50%. The film is obtained through biaxially stretching.